1. Field of the Invention
The present invention relates to the post-forming of jointing formations—such as sockets or flanges—on pipes of plastics material, in which the material undergoes local bending during the post-forming process. The invention has particular application to the formation of ring grooves in the socketing of molecularly oriented plastic pipe and is described here in that context, but is not limited to that application.
2. Description of Related Art
Means of joining lengths of plastic pipe together are many and varied. Many of them involve the reforming of the end of the pipe by reheating and shaping to some desired profile to provide a means of mating with the opposing end of the next pipe through a clamp, socket, or coupling device. The art of forming sockets (also called bells) on plastics pipes is well established, and there are numerous processes and methods in the literature.
It is well established that molecular orientation of plastics can provide enhanced mechanical properties, and such materials are commonly used for plastics pipes. Orientation is achieved by drawing or stretching the material under appropriate conditions of temperature, such that a strain (i.e. deviation from the originally formed dimensions) is induced in the plastics material to cause alignment of the molecules, and cooling the material while drawn to lock in that strain. A number of methods have been proposed whereby this principle is applied to plastic pipes, in particular in order to enhance the burst strength under internal pressure by circumferential and/or axial drawing. Examples of such a method are those described in WO 90/02644 and WO2004/089605, which describe processes for production of biaxially (i.e. circumferentially and axially) oriented pipe.
In the case of oriented thermoplastics, the most common piping materials, the reforming of oriented material is not so simple, since the material will tend to revert if reheated, that is to say, the oriented molecular structure, which is itself created by a deformation process, will be lost. Further, the deformation processes applied to the socket may alter the orientation level in such a way that the strength or other mechanical properties of the material are adversely affected. This latter effect is discussed later in more detail with reference to FIG. 1.
Oriented PVC pipe is usually jointed by an integral socket formed on the end of the pipe as described in WO90/15949, U.S. Pat. No. 5,928,451, WO02/09926, and UK1,432,539. In these methods, the end of the pipe is enlarged to form a socket, either by mechanical or hydraulic means to force the material to conform to an external mould or internal mandrel.
WO90/15949 relates to an integral socket arrangement for a circumferentially or biaxially oriented plastics pipe, which was characterised by applying a differential axial draw ratio to the socket and the body of the pipe to produce the desired relative thickness and properties. In particular, that process provided a circumferentially or biaxially oriented plastics pipe comprising a body with an integral socket at one end thereof, with the socket wall having lesser axial draw than the body of the pipe.
Additionally, it is often desired to use a sealing ring to seal the connection formed by insertion of the pipe end into the enlarged socket. To accommodate this sealing ring, the socket will include an internal ring groove, typically formed by stretching the socket end over a specially-shaped mandrel enlarged about a circumferential locus to form an annular groove that will house a ring gasket of elastomeric material for sealing purposes.
In WO97/10942, there was described a method for creating a socket and ring groove in the end of a molecularly oriented pipe, including the step of heating the region of the socket in which the ring groove is to be formed to above the glass transition of the temperature of the material whilst maintaining the remainder of the socket below the glass transition temperature.
In the forming process, bending occurs at points of changes in direction of the surface, generating tensile or compressive strains in the material at that point. These strains add to or subtract from the strains generated in the orientation process and give rise to increased or decreased orientation. The bending stresses caused in formation of the ring groove have been found to modify the localised axial draw of the material in the vicinity of the ring groove, compared to the axial draw of the remainder of the socket. On the inside of the bend (i.e. the concave surface of the bend), the material of the ring groove is compressed (resulting in less axial draw), while on the outside of the bend (i.e. the convex surface of the bend) the axial draw will be increased. Along the neutral bending axis—extending approximately along the midpoint of the material section—the axial draw will be essentially unaltered.
This localised modification of axial draw can in some instances be a cause of weakness of the pipe socket through the ring groove.